A furnace is an expensive investment, but with the rising cost of gas and the frigid winter this year, it may be time to take the plunge. If you have an older furnace, it may not be that long before a more efficient furnace pays for itself, and a newer heater will almost certainly keep you and your family warmer through those cold winter nights.

Is your furnace more than 15 years old?

If it is, there's a good chance that it's time to replace it. According to Energy Star, by replacing a conventional furnace with an Energy Star-rated furnace you will be heating your home with about 15 percent more efficiency. In order to check how long it will take for your new furnace to pay for itself, first add up your heating bills for the last year. Once you have your total yearly cost, divide by 100 and multiply by 15 - this is how much you will save every year with your new furnace. All you have to do now is divide the cost of the new furnace by your savings and you have the number of years it will take to pay for itself. It's worth noting that if your furnace is under 15 years old, it's usually more cost effective to pay for repairs even with the added efficiency.

If your energy bills are going up unexpectedly or your house is unusually cold, it's possible that your furnace has become less efficient over time. In this case, it's possible that repairs will help to get back some of the lost efficiency - often the solution is as simple as fixing a problem with your ducts, lack of insulation, or improperly installed equipment. If your furnace is near the end of its expected life, however, it may make more sense monetarily to simply replace it with a newer model.

When can I just repair?

There are several major malfunctions related to gas furnaces that can be fixed comparatively cheaply and simply according to Consumer Reports. If you are getting poor airflow, make sure you check the air filter, as this may be the cause of the problem. If your house is consistently too hot or too cold, the thermostat may be to blame. In this case, the answer may be as easy as replacing the batteries, or fixing a small malfunction like a loose wire. Check to see if any fuses have been tripped - it's possible that power could be cut to the circuit board or the fan. If your furnace is particularly noisy, you might just have an undersized duct system.

While you're checking on your furnace, it's worth thinking about whether it seems like your house has excessive dust. Old pipes can end up pulling dust from attics or basements and distribute it throughout the house. If this is the case, sealing these ducts is a low cost way to fix the problem and increase the efficiency of your heating system.

What should I do first?

The first thing you should do is to get a home inspection. A home inspection professional can come once a year and tell you whether your heating system would benefit from repairs and when it's time to replace your system. A Consumer Reports survey found that heating professionals reported human error - like improper maintenance or installation - as being the reason for a malfunction twice as often as defective equipment. Getting a home inspection will catch any of these problems and ultimately save you money.